1. Field
Adhesive compositions suitable for use in polymer-to-metal, for example elastomer-to-metal such as rubber-to-metal, bonding applications are provided. One aspect provides compounds suitable for use in such adhesive composition rubber to metal bonding applications.
2. Brief Description of Related Technology
Polymer to metal and in particular rubber to metal bonding has been practiced for many years. There are many applications for formulations which achieve polymer or rubber to metal bonding. Rubber to metal bonding is widely used to bond different metals to a natural or synthetic rubber. Polymer to metal bonding is carried out for many reasons.
One aspect of rubber to metal bonding is to combine the structural strength of the metal with the elastomeric properties of the rubber.
Accordingly, metal and polymers such as rubber are often bonded to each other for impact absorption applications, such as in bearings, wheels, shock absorbers, moving arms etc. Such components can be utilised on a very small scale, for example in PC components or on a very large scale for example in constructions such as bridges and buildings. Noise reduction may also be achieved by utilising metal to rubber bonding. It is accepted that tremendous forces can be experienced by any component that comprises metal and rubber bonded together. Accordingly, it is desirable to provide metal to rubber bonding, which can withstand significant forces, such a compressive or extensive pressures including shocks without having the metal or the rubber separate from each other. There are many other applications including in tyre production where internal wire reinforcements for the tyre are bonded to the rubber of the tyre.
Generally, an adhesive formulation is provided to bond a selected rubber to a selected metal substrate.
Traditional rubber-to-metal bonding technology incorporates a two-step system, where in a first step a primer is applied and thereafter in a second step an adhesive is applied. The primer ordinarily consists of solutions or suspensions of chlorinated rubber and phenolic resins containing reactive groups, and also pigments such as titanium dioxide, zinc oxide, carbon black etc. The primer is generally applied as a thin layer onto a treated (cleaned) surface of a metallic component such as treated steel component for example a component that has been grit blasted or chemically treated.
The adhesive ordinarily consists of a large range of rubber materials and cross-linkers. These include, but are not restricted to, chlorinated and bromochlorinated rubbers, aromatic nitrosobenzene compound and bismaleimide as cross-linkers, xylene, perchloroethylene and ethylbenzene as solvents, and also some lead or zinc salts. The adhesive layer is generally the link between the primed metal and the rubber.
Common cross-linkers that have been employed in rubber-to-metal bonding technology are aromatic nitroso compounds, such as p-dinitrosobenzene.
In the many fields where rubber to metal bonding is employed there are numerous opportunities to address several types of problems, including bond strength and durability, sample preparation and ease of application of the adhesive, single coat versus two-coat systems, reduced toxicity and improved properties for the environment, amongst others.
Many formulations for rubber to metal bonding exist. For example silanes have been used as corrosion inhibitors and as rubber-to-metal bonding adhesion promoters. Reducing VOCs (volatile organic compounds), and reducing the costs, such as those associated with cobalt salts, which are used as an adhesion promoter in certain systems is desirable for example as set out in the following US Patent Application Publication Nos. US2007/0056469A1 and US2005/0079364A1, and U.S. Pat. No. 6,756,079B2 and U.S. Pat. No. 6,409,874. Specific silanes noted are bis(trimethoxypropyl)amine and bis(triethoxypropyl)tetrasulfide, generally in use together in a ratio of 1:3 respectively, in an ethanol/water solution. It is noteworthy that the silanes are thus hydrolysed, either separately or together, prior to application to the metal.
International (PCT) patent publication No. WO2004/000851 to Qinetiq Ltd, describes a process for silane hydrolysis with a good degree of control. The process is carried out in solution in the presence of water and a catalyst. International (PCT) patent publication No. WO2004/078867 to Lord Corporation describes a single coat solvent-based adhesive designed to bond thermoplastic elastomers containing an alkoxy silane/urethane adduct and a chlorinated polymer. Methods of synthesis and formulation are described within this patent document. U.S. Pat. No. 4,031,120 to Lord Corporation describes a composition comprising an isocyanate functional organosilane, in combination with a polyisocyanate and an aromatic nitroso compound. The resulting system is described as a one-coat adhesive for bonding a variety of elastomeric materials to metals and other substrates.
Generally it is desirable that bonding is achieved during a vulcanisation step like compression moulding, transfer moulding, injection moulding and autoclave heating, for example with steam or hot air. For example semi-solid rubber can be injected into a mould. The semi-solid rubber is then cross-linked into a fully cured rubber and the bond with the substrate is formed at the same time.
Certain requirements of the curing system are desirable. This includes, ease of processing; stability (for example avoiding sedimentation), ease of application, fast drying (to allow handling without fouling); good wetting properties, and good curing strengths. Curing should be achieved independently of the type of elastomer (rubber) employed and also independently of the type of substrate. It will be appreciated that some rubbers are blended materials and accordingly it is desirable that good curing is achieved with such blended materials. Suitably consistent curing is achieved under various process parameters. Durability it also desirable. For example the rubber/metal joint, should be durable under relatively high mechanical stress for example under high pressure and even if immersed in an aggressive atmosphere, for example a hot liquid such as oil.
Notwithstanding the state of the technology it would be desirable to provide a compound and composition to bond polymeric substrates to metal substrates that remedies some or all of the deficiencies of that which was known before and/or provides alternatives to the existing technologies so that consumers have more possibilities from which to choose.